Bump MSRV to 1.91 (#711)

Not sure why things have been working so far. Apparently, `TypeId::of` in `const` contexts has only been stabilized in 1.91.

Author: filmor

.clippy.toml

@@ -1 +1 @@
-msrv = "1.70"
+msrv = "1.91"

README.md

@@ -57,7 +57,7 @@ rustler::init!("Elixir.Math");
 
 #### Minimal Supported Rust Version (MSRV)
 
-Rustler currently has a minimal supported Rust version (MSRV) of 1.70. This
+Rustler currently has a minimal supported Rust version (MSRV) of 1.91. This
 is the configured version in `.clippy.toml`.
 
 #### Supported OTP and Elixir Versions

rustler/Cargo.toml

@@ -7,7 +7,7 @@ authors = ["Hansihe <me@hansihe.com>"]
 license = "MIT/Apache-2.0"
 readme = "../README.md"
 edition = "2021"
-rust-version = "1.70"
+rust-version = "1.91"
 
 [features]
 big_integer = ["dep:num-bigint"]